I would recommend bouncing around to as many muds as you can to find your niche.

Do you just want to kill other players? Plenty of PK muds.
Do you like story-telling and deep character interaction? Try an RPI.
Are you a socialite looking for a chat-room with tons of grinding? Try aardwolf or something.

Best bet is to go down the list of top-20 Muds and just try each one for a week before moving on to the next, there are tons of different worlds and systems in place, so each mud is different in the top-20. Stock muds I would recommend staying away from as they are a dime a dozen and do not have much originality nor playerbase.

I personally really enjoy MUDing and spend a lot of time doing it! It really allows for an active imagination to create whatever they want to see. It's much better for me than any graphical game could ever be.